Sleep is a state of brain activity defined as unconsciousness from which a person can be aroused by sensory or other stimuli (Arthur C. Guyton, Textbook of Medical Physiology 659 (8th ed. 1991)). While asleep, a person typically goes through two alternating states of sleep, rapid eye movement (REM) sleep and non-REM sleep. Non-REM sleep is comprised of four sleep stages. Stage 1 (S1) is a state of drowsiness or transition between wake and sleep. Stage 2 (S2) is a state of light sleep. Stage 3 (S3) and stage 4 (S4) are stages of deep sleep. REM sleep occurs about 80 to 100 minutes after falling asleep, and is characterized by high frequency EEG activity, bursts of rapid eye movement, and heightened autonomic activity. Sleep progresses in a cycle from stage 1 through stage 4 to REM sleep. A person typically experiences four to six REM periods per sleep period.
Sleep disorders such as sleep apnea, restless legs syndrome, and narcolepsy, inevitably result in sleep deprivation, which among other things, interferes with work, driving, and social activities. Various neurological and psychiatric conditions are also associated with disruptions of normal sleep patterns. For example, insomnia or oversleeping may occur in individuals with depression, and those experiencing a manic upswing characteristic of bipolar disorder may not sleep at all. Furthermore, because REM sleep increases sympathetic nervous system activity, myocardial ischemia or arrhythmia may be triggered during REM sleep in individuals with preexisting heart pathology. Thus, the detection and characterization of sleep states is important in the evaluation and treatment of many medical conditions.
Currently, information about sleep stages is obtained using polysomnography, a procedure in which data is acquired related to various body activities, including EEG waveforms, cardiac muscle activity, and breathing. This type of evaluation usually is conducted in a sleep laboratory over one or more nights, and thus is not convenient when it is desirable to monitor a patient over an extended period of time. There are ambulatory sleep monitoring systems, however continuous and extended sleep monitoring can nonetheless be inconvenient because these systems must be applied to the patient and adjusted prior to every sleep episode. Furthermore, many known devices and systems for assessing EEG waveforms (e.g., using time-domain and frequency-domain analysis) require more computational ability than can be easily included in an implantable device.
Accordingly, it would be desirable to have an implantable device capable of continuously monitoring sleep. It would also be desirable to have implantable devices that are capable of detecting different stages of sleep. Similarly, it would be desirable to have implantable devices with monitoring and detecting functions for evaluating and treating various medical conditions associated with disturbed sleep patterns.